ath9k: remove a bogus WARN_ON

On embedded hardware it's normal to not have a PCI device for the PCI
bridge that the wifi card is attached to. pdev->bus->self will be
NULL in that case. In that case, simply return without emitting an
useless kernel stack trace.
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/pci.c

@@ -126,7 +126,7 @@ static void ath_pci_aspm_init(struct ath_common *common)
 		 * Both upstream and downstream PCIe components should
 		 * have the same ASPM settings.
 		 */
-		if (WARN_ON(!parent))
+		if (!parent)
 			return;
 
 		pos = pci_pcie_cap(parent);
@@ -137,7 +137,7 @@ static void ath_pci_aspm_init(struct ath_common *common)
 		return;
 	}
 
-	if (WARN_ON(!parent))
+	if (!parent)
 		return;
 
 	pos = pci_pcie_cap(parent);